1. Field of the Invention
The present invention relates to a toner, particularly for use in a developer for developing an electrostatic latent image by electrophotography, electrostatic recording, electrostatic printing and the like, and to a method of preparing the toner.
2. Discussion of the Background
In electrostatic latent image formation in the methods of electrophotography, electrostatic recording, electrostatic printing and the like, a developer is adhered to an image bearer, such as a photoreceptor on which an electrostatic latent image is formed in the development process. The developer is then transferred therefrom onto a transfer medium, such as a transfer paper, in a transfer process; and then fixed on the transfer medium in a fixing process. The developer can be generally a two-component developer formed of a carrier and a toner; or a one-component developer without a carrier, i.e., a magnetic or a non-magnetic toner, respectively.
Conventionally, a dry toner is formed by kneading a toner binder, such as a styrene resin or a polyester resin, with a colorant upon application of heat to form a kneaded mixture, cooling the mixture to solidify the mixture and pulverizing the solidified mixture.
The particle diameter of the toner is downsized to produce high-definition and high-quality images. However, a toner formed by the conventional kneading and pulverizing method has an amorphous particle form and cannot be classified. This is because ultrafine particles having a strong adherence keep adhering to the toner having a desired particle diameter, even after a classifying process. In an image forming apparatus, such ultrafine particles adhere to a carrier and apparatus parts and are fixed thereon due to being stirred with the carrier in the image developer, and due to contact stress from a developing roller, a toner feeding roller, a layer-thickness regulation blade and/or a frictional-charged blade. In the meantime, fluidizer is buried in the surface of the toner, resulting in deterioration of the quality of the resultant images. In addition, amorphous toner having low fluidity as a powder needs a large amount of fluidizer and the filling rate thereof into a toner bottle is so low that the amorphous toner is one of the impediments to downsizing of the apparatus.
Therefore, toners having a small particle diameter are not yet fully utilized. Further, the kneading and pulverizing method has a particle diameter limit, and is unable to further effectively downsize the particle diameter beyond that limit.
Further, to produce full-color images, the transfer process for transferring an image formed of multiple color toners from a photoreceptor to a transfer medium and a paper is complicated. Because of its poor transferability, amorphous pulverized toner is consumed in a larger amount to achieve the same level of image formation.
Accordingly, demands for reducing the running costs and producing high-definition images, without image omission, by improving transferability of the toner to decrease the consumption thereof, are increasing. This is because better transferability of the toner can dispense with the need for a cleaning unit to remove untransferred toner from a photoreceptor and a transfer medium. Therefore the apparatus can be downsized, the cost can be reduced and there is minimal waste toner. To improve such disadvantages due to the shapes, methods of producing toners having various shapes have been proposed, such as suspension polymerization methods and emulsion polymerization condensation methods.
Japanese Laid-Open Patent Publication No. 7-152202 discloses a polymer dissolution suspension method accompanied with a volume contraction. The method includes dispersing or dissolving toner materials in a volatile solvent, such as a low-boiling organic solvent, to form a dispersion or a solution; emulsifying the dispersion or solution in a water medium including a dispersant, to be a droplet; and removing the volatile solvent therefrom.
Differently from the suspension polymerization methods and emulsion polymerization condensation methods, the polymer dissolution suspension method accompanied with a volume contraction can use general resins such as a polyester resin effectively used for full-color images needing transparency and smoothness after fixed. However, a dispersant used in the method strongly adheres to the surface of the toner and is difficult to remove by washing, and therefore the chargeability of resultant toner deteriorates, i.e., the resultant toner is charged low, slowly charged and strongly affected by humidity.
Japanese Laid-Open Patent Publication No. 11-149179 discloses a method of decreasing the viscosity of the dispersed phase using a low-molecular-weight resin in the polymer dissolution suspension method to make the emulsification easier, and performing an inter-particle polymerization to improve the fixability of the resultant toner. However, when a functional group, particularly an isocyanate compound is used in the inter-particle polymerization, chargeability of the resultant urethane and urea group largely affects the chargeability of the resultant toner.
To solve such problems, Japanese Laid-Open Patent Publication No. 2001-343786 discloses a method of directly preparing a toner by polymerizing a metal oxide of aromatic oxy carboxylic acid, a colorant, a low-softening point material and a composition of polymerizing monomers having a polar resin in a water-medium, wherein the metal oxide of aromatic oxy carboxylic acid, colorant, low-softening point material and polar resin are dispersed in the composition of polymerizing monomers; the metal oxide of aromatic oxy carboxylic acid which is soluble in an alkaline aqueous solution having a pH of from 9 to 13 is further added to the water-medium in a process of polymerization or granulation to perform a polymerization in the water-medium having a pH of from 4.5 to 9.0; the pH thereof is readjusted to have 9 to 13; and the metal oxide of aromatic oxy carboxylic acid is separated out on the surface of the toner by performing an acid treatment in the water-medium having a pH of from 1.0 to 2.5. Although the chargeability of resultant toner improves, the metal oxide of aromatic oxy carboxylic acid freely present on the surface of the toner noticeably contaminates a charging member such as a carrier. The charge quantity of the toner largely deteriorates with time and the toner cannot be practically used.
Japanese Laid-Open Patent Publication No. 11-84726 discloses a method of adding a boric acid aqueous solution or a metal salt aqueous solution to an agglomerate of a latex emulsion having a colorant at 30 to 95° C. to prepare a mixture; adding a salicylic acid or a catechol thereto after adding a base thereto to have a pH of from 9 to 12 to chemically reform the surface of the agglomerate of a latex emulsion. However, only zinc is used in the Examples, and the reaction temperature is as high as 85° C.
In this case, the zinc sulfate is a bivalent metal and the zinc ion has a tetracoordination, and the unitable salicylic acid or catechol has one molecule. As a result of an investigation of the present inventors, when a unitable organic acid has one molecule, i.e., when a bivalent metal is used, a the resultant toner does no have desired chargeability. Since the salicylic acid is added when the pH is from 9 to 12 and the temperature is as high as from 30 to 95° C. Further, since the pH is maintained until the reaction is completed, the metallic compound cannot be considered to sufficiently react. In addition, it is a problem that a large amount of heat is applied to the toner. Namely, the method is unable to prepare a low-temperature fixable toner having a low glass transition temperature, or take a long time to complete the reaction when the reaction temperature is decreased to 30° C. or less.
For a conventional pulverized toner, a functional organic compound called a charge controlling agent (CCA) is included in a toner composition before kneaded, and the toner composition including the CCA is uniformly dispersed by kneading upon application of heat and is pulverized such that a specific amount of the CCA is present on a surface of the toner to have a desired charge controllability.
Even when the CCA used for the conventional pulverized toner is included in the toner of the present invention, an effect thereof is apparently low and falls far short of a desired target.
It is supposed that this is because when the functional organic compound has a high lipophilic property, the functional organic compound is dispersed in the toner and scarcely present on the surface thereof, and therefore the CCA exerting its effect when present on the surface thereof in a specific amount cannot exert its effect at all or can scarcely exert its effect.
When the functional organic compound has a high hydrophilicity, the functional organic compound gradually moves to an aqueous phase while the toner is prepared, and is supposed to be scarcely present on the surface thereof. In this case, since the functional organic compound is neither present on the surface thereof nor therein, the charge controlling effect is not exerted at all.
It is quite difficult to make the functional organic compound selectively present on the surface of the toner by the conventional technologies, and therefore it is quite difficult to impart a sufficient charge controlling effect thereto.
Because of these reasons, a need exists for a toner having a high average of charged level, a high stability thereof, a high speed of being charged, a sharp particle diameter distribution, a spherical shape, a high melting viscosity, and which is not affected by an environment such as humidity and temperature.